The present invention relates generally to processing and displaying digital documents and, more particularly, to processing images in a raster image processor using transparency information.
Digitally stored documents are often represented in an output-device independent language known as a page description language (PDL). Well known PDLs include the POSTSCRIPT language and the PORTABLE DOCUMENT FORMAT (PDF) language. Both POSTSCRIPT and PORTABLE DOCUMENT FORMAT are trademarks of Adobe Systems Incorporated, San Jose, Calif. In a PDL representation, a document may be described in terms of a list of instructions and resources. Instructions, for example, include commands to draw a line between two points or to fill a region with a specified color. Resources, for example, include images (raster representations of displayable objects), color spaces, and font specifications.
The transparency of a graphical object indicates the extent to which an underlying object may be seen through it. Here, the term object is used to denote any graphical entity such as an image, a font, or any other printable feature. In graphical processing, an object's transparency is implemented through a technique known as blending. Blending involves combining a foreground color, associated with a foreground graphical object, and a background color, associated with a background graphical object, to create a third color. Blending can give an object the appearance of being translucent.
In conventional graphical processing systems, blending generally is implemented in a special purpose application program executing on a general purpose computer. In this environment, blending operations interrupt the execution of the application program and consume the computer processor's time and memory resources. In addition, these special purpose applications are not typically capable of optimizing their output to a specific output device.